Field of the Invention
The invention concerns an electric coil apparatus of the type having at least two subcoils made of a superordinate doubly-connected tape conductor, the tape conductor having at least two conductor branches and the subcoils each being wound from one of these conductor branches.
Description of the Prior Art
Superconducting coils are used to generate strong, homogeneous magnetic fields, with the superconducting coils being operated in sustained short-circuit current mode. Homogeneous magnetic fields with magnetic flux densities between 0.5 T and 20 T are needed, for example, for nuclear magnetic resonance spectroscopy (NMR spectroscopy) and for magnetic resonance imaging. These magnets are typically charged via an external current circuit and are then separated from the external power source as, in the resulting sustained short-circuit current mode, an almost loss-free current flow occurs via the superconducting coil. The resulting strong magnetic field is particularly stable over time as it is not affected by the noise contributions of an external current circuit.
Where known winding technologies are used, one or more superconducting wires are wound on supporting bodies, different wire segments being linked to one another via wire connections with optimally low ohmic resistance or via superconducting connections. For classic low-temperature superconductors such as NbTi and Nb3Sn with transition temperatures below 23 K, technologies exist for manufacturing superconducting contacts for linking wire segments and for connecting windings to a superconducting sustained current switch. The superconducting sustained current switch is thereby part of the current circuit of the coil and is placed in a resistive conducting state by heating, in order to feed in an external current. After switching off the heating and cooling down to the operating temperature, this part of the coil also becomes superconducting again.
High-temperature superconductors, also called high-Tc superconductors (HTS), are superconducting materials with a transition temperature above 25 K, and above 77 K in some material classes such as cuprate superconductors, in which the operating temperature can be achieved by cooling with cryogenic materials other than liquid helium. HTS materials are particularly attractive for the manufacture of magnetic coils for NMR spectroscopy and magnetic resonance imaging, as some materials have high upper critical magnetic fields of over 20 T. Due to the higher critical magnetic fields, the HTS materials are in principle more suitable than the low-temperature superconductors for generating high magnetic fields of, for example, over 10 T.
A problem in the manufacture of HTS magnetic coils is the lack of suitable technologies for the manufacture of superconducting HTS connections, in particular, for second-generation HTS, known as 2G-HTS. The 2G-HTS wires occur typically in the form of flat tape conductors. If resistive contacts are inserted between the superconducting tape conductors the losses in the coil can no longer be ignored, and the generated magnetic field drops off noticeably over a period of a few hours or days.
In DE 10 2010 042 598 A1, a superconducting MR magnet arrangement is described that has a superconducting tape conductor that is provided in the longitudinal direction with a slit between the two ends such that the superconducting tape conductor forms a closed loop enclosing the slit. In the magnet arrangement, the superconducting tape conductor is wound on at least one double coil made of two subcoils which are arranged rotated relative to one another such that they generate a predetermined magnetic flux in a measurement volume.
A method for manufacturing a coil with a slitted tape conductor is also disclosed in the non-prepublished German patent application under file number 102013207222.8. Here, the two conductor branches of the tape conductor are simultaneously wound on two adjacent parts of a winding support.
In these known coil apparatuses with slitted, doubly connected tape conductors, the two subcoils are wound simultaneously, such that the corresponding superordinate surfaces of the superordinate tape conductor are oriented radially in the same direction. Thus, a superordinate surface of the tape conductor is either oriented uniformly toward the coil interior via a pair of simultaneously wound subcoils or uniformly toward the coil exterior. By means of this uniformly directed and simultaneous winding of the two subcoils, the individual conductor branches can be routed adjacent to one another during winding without any significant torsion of the tape conductor. If after winding the subcoils are arranged relative to one another such that their common magnetic fields reinforce one another, they have to be turned counter to one another so that significant torsion then occurs at least locally in sub-areas of the conductor branches. A disadvantage in these known coil apparatuses is thus that the conductor branches are twisted in places when the finished coil apparatus is operated. This causes a mechanical straining of the conductor branches during operation, which can lead to impairment of the quality of the superconducting layer. In order, as far as possible, to prevent any such impairment of the superconductor, additional space is required for the protruding ends of the conductor branches, as the strain due to the twisting can be alleviated if the torsion extends over a longer section of the conductor. This additional space requirement is, however, also disadvantageous, as coil arrangements for electric coil apparatuses are generally supposed to be of as compact a design as possible.